An idiot to select carp (7)
I believe the answer is:
nitpick
'carp' is the definition.
(both can mean to find fault with something)
'an idiot to select' is the wordplay.
'an idiot' becomes 'nit' (I've seen this before).
'to select' becomes 'pick' (picking is a kind of selecting).
'nit'+'pick'='NITPICK'
(Other definitions for nitpick that I've seen before include "Make petty criticisms" , "Find little fault" , "Have little complaint" , "Make petty criticism" , "Raise petty objections" .)